In the manufacture of integrated circuits structuring in the vertical direction of the semiconductor substrate has, due to modern doping techniques and highly advanced methods for the deposition of layers on semiconductor surfaces, developed to such an extent that the possibilities of structuring in the horizontal direction have been left far behind. Intensive efforts are, therefore, made to scale down the structure of integrated circuits in the lateral dimensions of the semiconductor disk. To this end, a change from whole-disk exposure to step-by-step exposure of identical circuits on a semiconductor disc is taking place. Parallel developments are seeking alternatives to optical lithography on which all methods for the manufacture of integrated circuits presently in use are based. Such alternatives are electron beam lithography and X-ray lithography in particular. Electron beam lithography has already been practically applied in the manufacture of masks. The direct treatment of semiconductor disks with electron beams is very complicated, however, and the costs are far too high in view of the low output. Moreover, experiences gathered in photolithography, e.g. in respect of the use of certain photosensitive layers, are no longer applicable in such basically new methods. X-ray lithography is in a very early experimental state, and its development is hindered by the lack of sufficiently intense X-ray sources as well as by the low efficiency of these sources and by complicated masking techniques.
Practical progress is most likely to be made by improving optical lithography, whereby local changes in the molecular structure of the photosensitive layer are obtained by local exposure of the layer. Hence, efforts are made to increase the resolving capability by employing so-called "deep" ultraviolet light (about 270 nm), which means that the limits drawn by diffraction effects are shifted. Operations in this wavelength range involve the troublesome work of developing conventional optical components, i.e. lenses, filters as well as photosensitive resists into components which are effective. A further disadvantage results from the fact that alignment operations, which are one of the main problems of all industrial lithographic methods, are most advantageously carried out by means of visible light. When ultraviolet is used as the exposure light, alignment operations either have to be carried out with visible light causing inaccuracies or inconveniences of troublesome and difficult operations with UV-detectors.
It is possible, in principle, to improve the resolving capability of a lens by increasing the opening angle. This possibility is, however, limited by the structure of the projection lens and, above all, by a problem which is characteristic of the lithography of structured surfaces. This problem is the vignetting, i.e. the shading of parts of the image-forming rays by projecting parts of the semiconductor surface. Due to this effect, the opening angle in photolithographic devices lies necessarily below the amount at which total reflection would occur at the boundary surface of the planar substrate. Consequently, measures for eliminating total reflection in order to increase the opening angle have not heretofore been taken into consideration.